1. Field of the Invention
The present invention relates to a high-frequency piezoelectric oscillator, and more particularly, to a high-frequency piezoelectric oscillator having en excellent stabilization characteristics with suppressing unwanted resonance.
2. Description of the Related Art
Conventionally, an odd-order overtone, for example, third, fifth, and seventh-overtone of a vibrator is utilized to obtain oscillation in high frequency. In order to obtain the overtone oscillation, a harmonic selection circuit having high negative resistance at a desired frequency is provided in the oscillation circuit. FIG. 22 illustrates one example of a conventional Colpitts type oscillator. A capacitor C11 that becomes a part of a load capacitance of the oscillation circuit is connected between a base and an emitter of a transistor TR11. A parallel resonance circuit consisting of a capacitor C12 and an inductor L11 is connected to the emitter of the transistor TR11. A parallel circuit of a capacitor 13 and an emitter resistor R11 are connected in series to the parallel resonance circuit, and are grounded. A base bias circuit consisting of a resistor RB11 and a resistor RB12 is connected to a base of the transistor TR11. A series circuit of a piezoelectric vibrator (X′tal) and a capacitor C14 is connected between the base of the transistor TR11 and the ground. A collector of the transistor TR11 and a power supply line (VCC) are connected together.
In the present circuit, an oscillation output cannot be obtained when a desired frequency becomes 600 MHz or above. That is, the resonance frequency of a parallel resonance circuit comprised of the capacitor C12 and the inductor L11 can be set to a desired level, however, when the frequency is 600 MHz or above, an impedance of the piezoelectric vibrator lowers due to an interelectrode capacitance C0 of the piezoelectric vibrator. Accordingly, sufficient negative resistance cannot be generated in an oscillation loop of the oscillation circuit. To overcome this difficulty, as shown in FIG. 23, the inductor L20 is inserted in parallel into the piezoelectric vibrator (X′tal). The interelectrode capacitance C0 is canceled by matching the parallel resonance frequency of the interelectrode capacitance C0 and the inductor L20 with the oscillation frequency. Since the parallel circuit of the interelectrode capacitance C0 and the inductor L20 prevents deterioration of the negative resistance of the oscillation circuit and provides high selectivity, a high frequency oscillation can be achieved.
In order to make clear a difference between the present invention and the conventional circuit, the circuit shown in FIG. 23 will be explained in further detail. According to the conventional circuit, capacitors C21 and C22 that form a part of the negative capacitor are connected between the base of the transistor TR21 and the ground. The connection point of the capacitors C21 and C22 is connected to an emitter of the transistor TR21, and is grounded via a resistor R21. A base bias circuit consisting of a resistor RB21 and a resistor RB22 is connected to abase of the transistor TR21. A parallel circuit of the piezoelectric vibrator (X′tal) and the inductor L20 is connected to a capacitor C23, and a series circuit of the parallel circuit and the capacitor is connected between the base of the transistor TR21 and the ground. Further, a collector of the transistor TR21 and the power supply line (Vcc) are connected.
FIG. 24 illustrates an equivalent circuit model of the conventional circuit shown in FIG. 23. In FIG. 24, the piezoelectric vibrator is indicated by the equivalent circuit comprising a reactance L1, capacitance C1, C0, and resistance R1, and an oscillation circuit is indicated by negative resistance −Rc and reactance Xc. FIG. 26 illustrates another equivalent circuit model of the conventional circuit in which a reactance of a parallel resonance circuit comprised of the capacitance C0 and inductance L0 shown in FIG. 24 is presented as X0.
Expressions for an oscillation condition are as follows.                                                                                           X                  c                                =                                ⁢                                  1                                      ω                    ·                                          C                      c                                                                                  ,                                                           ⁢                              …                ⁢                                                                   ⁢                                  X                  0                                                                                                                        =                                ⁢                                  1                                      ω                    ·                                                                  C                        0                                            ⁡                                              (                                                                                                            ω                              0                              2                                                                                      ω                              2                                                                                -                          1                                                )                                                                                                        ,                                                           ⁢                              …                ⁢                                                                   ⁢                                  C                  α                                                                                                                                          =                                    ⁢                                                            C                      0                                        ⁡                                          (                                              1                        -                                                                              ω                            0                            2                                                                                ω                            2                                                                                              )                                                                      ,                                                                   ⁢                                                      …                    ⁢                                                                                   ⁢                                          C                      L                                                        =                                      -                                          1                                              ω                        ·                                                  X                          L                                                                                                                    ,                            ⁢                                                                                                       (        1        )                                                                                                      …                  ⁢                                                                           ⁢                                      X                    c                                                  =                                ⁢                                  1                                      ω                    ·                                          C                      c                                                                                  ,                                                           ⁢                              …                ⁢                                                                   ⁢                                  X                  0                                                                                                                        =                                ⁢                                                                            1                                              ω                        ·                                                  C                          0                                                                                      ⁢                                                                                   ⁢                                          (                                                                                                    ω                            2                                                                                ω                            0                            2                                                                          -                        1                                            )                                        ⁢                                                                                   ⁢                    …                    ⁢                                                                                   ⁢                                          R                      L                                                        =                                                                                    -                                                  R                          c                                                                    ⁢                                              X                        0                        2                                                                                                            R                        c                        2                                            +                                                                        (                                                                                    X                              0                                                        -                                                          X                              c                                                                                )                                                2                                                                                                        ,                                                           ⁢                              …                ⁢                                                                   ⁢                                  X                  L                                                                                                        =                            ⁢                                                                    X                    0                                    ⁢                                      {                                                                  R                        c                        2                                            -                                                                        X                          c                                                ⁡                                                  (                                                                                    X                              0                                                        -                                                          X                              c                                                                                )                                                                                      }                                                                                        R                    c                    2                                    +                                                            (                                                                        X                          0                                                -                                                  X                          c                                                                    )                                        2                                                                                                          (        2        )                                                                                                                          …                    ⁢                                                                                   ⁢                                          R                      1                                                        +                                      R                    L                                                  =                0                            ⁢                                                                                                                                                                             ·                    ω                                    ⁢                                                                           ⁢                                      L                    1                                                  +                                  1                                      ω                    ·                                          C                      1                                                                      +                                  X                  L                                            =              0                                                          (        3        )            
FIG. 25 illustrates a result of carrying out a simulation about characteristics of negative resistance Rc and capacitance Cc of a conventional Colpitts oscillation circuit. The axis of ordinates represents negative resistance, and the axis of abscissas represents frequency. From FIG. 25, it is clear that negative resistance does not occur when the frequency is about 400 MHz or below. However, negative resistance considerably occurs over 400 MHz frequencies. It can be seen that negative resistance occurs sufficiently at 2 GHz.
Impedance ZL is obtained and Exps. (4) and (5) are obtained based on the equivalent circuit shown in FIG. 26. An Exp. (6) that shows a relationship between the resistance RL and the reactance XL shown in FIG. 27 is obtained from ZL.                                                                                           …                  ⁢                                                                           ⁢                                      Z                    L                                                  =                                                      j                    ⁢                                                                                   ⁢                                                                  X                        0                                            ⁡                                              (                                                                              -                                                          R                              c                                                                                -                                                      j                            ⁢                                                                                                                   ⁢                                                          X                              c                                                                                                      )                                                                                                                        -                                              R                        c                                                              -                                          j                      ⁢                                                                                           ⁢                                              X                        c                                                              +                                          j                      ⁢                                                                                           ⁢                                              X                        0                                                                                                        ,                                                           ⁢                                                …                  ⁢                                                                           ⁢                                      X                    c                                                  =                                  1                                      ω                    ·                                          C                      c                                                                                  ,                                                           ⁢                              …                ⁢                                                                   ⁢                                  X                  0                                                                                                                        =                                  1                                      ω                    ·                                                                  C                        0                                            ⁡                                              (                                                                                                            ω                              0                              2                                                                                      ω                              2                                                                                -                          1                                                )                                                                                                        ⁢                                                                                                       (        4        )                                                                    …              =                                                                                                         ⁢                                                            X                      0                                        ⁡                                          (                                                                        X                          0                                                -                                                  j                          ⁢                                                                                                           ⁢                                                      R                            c                                                                                              )                                                                                                            -                                          R                      c                                                        +                                      j                    ⁢                                                                                   ⁢                                          (                                                                        X                          0                                                -                                                                                                   ⁢                                                  X                          c                                                                    )                                                                                                                                              =                                                                                                         ⁢                                                                                    X                        0                                            ⁡                                              (                                                                              X                            c                                                    -                                                      j                            ⁢                                                                                                                   ⁢                                                          R                              c                                                                                                      )                                                              ⁢                                          {                                                                        -                                                      R                            c                                                                          -                                                  j                          ⁡                                                      (                                                                                          X                                0                                                            -                                                              X                                c                                                                                      )                                                                                              }                                                                                                            R                    c                    2                                    +                                                            (                                                                        X                          0                                                -                                                                                                   ⁢                                                  X                          c                                                                    )                                        2                                                                                                                                          =                                                                                                                                                 ⁢                                                                        -                                                                                    X                              0                                                        ⁡                                                          (                                                                                                X                                  c                                                                -                                                                  j                                  ⁢                                                                                                                                           ⁢                                                                      R                                    c                                                                                                                              )                                                                                                      ⁢                                                  {                                                                                    R                              c                                                        +                                                          j                              ⁡                                                              (                                                                                                      X                                    0                                                                    -                                                                      X                                    c                                                                                                  )                                                                                                              }                                                                                                                                    R                        c                        2                                            +                                                                        (                                                                                    X                              0                                                        -                                                                                                                   ⁢                                                          X                              c                                                                                )                                                2                                                                              ⁢                                                                           ⁢                  …                                            ⁢                                                                                                                                       =                                                                            -                                                                                           ⁢                                                                        X                          0                                                ⁡                                                  [                                                                                                                    X                                c                                                            ⁢                                                              R                                c                                                                                      +                                                                                                                   ⁢                                                                                          R                                c                                                            ⁡                                                              (                                                                                                      X                                    0                                                                    -                                                                      X                                    c                                                                                                  )                                                                                      +                                                          j                              ⁢                                                              {                                                                                                                                            X                                      c                                                                        ⁡                                                                          (                                                                                                                        X                                          0                                                                                -                                                                                  X                                          c                                                                                                                    )                                                                                                        -                                                                      R                                    c                                    2                                                                                                  }                                                                                                              ]                                                                                                                                    R                        c                        2                                            +                                                                        (                                                                                    X                              0                                                        -                                                                                                                   ⁢                                                          X                              c                                                                                )                                                2                                                                              ⁢                                                                           ⁢                  …                                            ⁢                                                                                                                       =                                                -                                                                           ⁢                                                            X                      0                                        ⁡                                          [                                                                                                    R                            c                                                    ⁢                                                      X                            0                                                                          +                                                  j                          ⁢                                                      {                                                                                                                            X                                  c                                                                ⁡                                                                  (                                                                                                            X                                      0                                                                        -                                                                          X                                      c                                                                                                        )                                                                                            -                                                              R                                c                                2                                                                                      }                                                                                              ]                                                                                                            R                    c                    2                                    +                                                            (                                                                        X                          0                                                -                                                                                                   ⁢                                                  X                          c                                                                    )                                        2                                                                                                          (        5        )                                                                                                      …                  ⁢                                                                           ⁢                                      R                    L                                                  =                                                                            -                                              R                        c                                                              ⁢                                          X                      0                      2                                                                                                  R                      c                      2                                        +                                                                  (                                                                              X                            0                                                    -                                                                                                           ⁢                                                      X                            c                                                                          )                                            2                                                                                  ,                                                           ⁢                              …                ⁢                                                                   ⁢                                  X                  L                                                                                                        =                                                                    -                                          X                      0                                                        ⁢                                      {                                                                                            X                          c                                                ⁡                                                  (                                                                                    X                              0                                                        -                                                          X                              c                                                                                )                                                                    -                                              R                        c                        2                                                              }                                                                                        R                    c                    2                                    +                                                            (                                                                        X                          0                                                -                                                                                                   ⁢                                                  X                          c                                                                    )                                        2                                                                                                                                          =                                                                                                    X                        0                                            ⁢                                              {                                                                              R                            c                            2                                                    -                                                                                    X                              c                                                        ⁡                                                          (                                                                                                X                                  0                                                                -                                                                                                                                   ⁢                                                                  X                                  c                                                                                            )                                                                                                      }                                                                                                            R                        c                        2                                            +                                                                        (                                                                                    X                              0                                                        -                                                                                                                   ⁢                                                          X                              c                                                                                )                                                2                                                                              ⁢                                                                           ⁢                  …                  ⁢                                                                           ⁢                                      R                    L                                                              ⁢                                                                                                                                       =                                                                            -                                              R                        c                                                              ⁢                                          X                      0                      2                                                                                                  R                      c                      2                                        +                                          S                      2                                                                                  ,                                                           ⁢                              …                ⁢                                                                   ⁢                                  X                  L                                                                                                                        =                                                                            X                      0                                        ⁢                                          {                                                                        R                          c                          2                                                -                                                                              X                            c                                                    ⁢                          S                                                                    }                                                                                                  R                      c                      2                                        +                                          S                      2                                                                                  ,                                                           ⁢                              …                ⁢                                                                   ⇐                            ,                                                           ⁢                                                ...                                                                           ⁢                  S                                =                                                      X                    0                                    -                                      X                    c                                                                                                          (        6        )            
FIG. 28 illustrates characteristics of a load resistance RL shown in FIG. 27. The axis of ordinates represents negative resistance Rc, and the axis of abscissas represents frequency. From FIG. 28, it is clear that, a largest negative resistance Rc is −300Ω at 600 MHz. The circuit shown in FIG. 27 constitutes an unwanted oscillation loop shown in FIG. 30, and the circuit oscillates in the resonance frequency of reactance X0+Xc=0. The oscillation loop includes negative resistance −Rc, and has no factor of anti-negative resistance. Therefore, oscillation occurs very easily. The Exp. (7) represents a frequency condition fω=0. FIG. 31 shows a frequency relationship.                               …          ⁢                                           ⁢          F                =                                            X              0                        +                          X              c                                =                                                    1                                  ω                  ·                                                            C                      0                                        ⁡                                          (                                                                                                    ω                            0                            2                                                    ω                                                -                        1                                            )                                                                                  -                              1                                  ω                  ·                                      C                    c                                                                        =            0                                              (        7        )            
FIG. 31 is a graph of unwanted resonance frequency when ft is Cc=3, 5, 10, 30, and 100 pF respectively in a condition that C0=3 pF, the parallel resonance frequency is f0=600 MHz, and the circuit negative resistance Rc=−100Ω. The frequency when X0−Xc=0 on each characteristic curve is unwanted resonance frequency.
FIG. 32 illustrates a relationship between unwanted resonance frequency and circuit capacitance. The axis of ordinates represents unwanted resonance frequency, and the axis of abscissas represents circuit capacitance. In the present circuit, X0 works as inductor at a low frequency side of the parallel resonance frequency, and this has a possibility of bringing about unwanted oscillation when the inductor is connected with the circuit capacitance at the circuit side. For example, when the parallel resonance frequency of the interelectrode capacitance C0 and the inductor L0 is set to 600 MHz and also when C0=3 pF, the parallel resonance frequency is 590 MHz when the circuit capacitance Cc=1 pF, and the parallel resonance frequency is 100 MHz when the circuit capacitance Cc=100 pF. However, from the above result of the simulation of negative resistance, there is a possibility that oscillation occurs at an unwanted resonance point in the vicinity of the parallel resonance frequency. Further, when an extension coil L1 is used to enlarge a variable range in the oscillation circuit loop, unwanted oscillation occurs in a wide band in connection with a large capacitance generated in the vicinity of the resonance point as shown in FIG. 29.
According to the conventional high-frequency oscillation circuit shown in FIG. 23, unwanted oscillation contributed by the oscillation circuit and the inductor L20 as explained above is easily occurred. Further more, when an extension coil for expanding a frequency variable range is inserted into the oscillation loop, unwanted oscillation occurs easily at the parallel resonance frequency defined by the interelectrode capacitance C0 and the inductor L20. Further, a high negative resistance cannot be obtained easily by the conventional oscillation circuits explained the above. Therefore, there are some report regarding experimental results of the high frequency oscillation circuit, but there is substantially no success in practical applications.